Engines may be operated at higher speeds and loads in response to an operator torque request. As engine speed and load increase, the exhaust gas temperature and exhaust mass flow rate can increase. If the engine is operated at a higher speed and load for an extended period of time, one or more engine components may experience at least some degradation. For example, if an engine is operated at a higher speed and load for an extended period of time, a temperature of a component such as a turbine or a catalyst may be elevated so that the component experiences at least some degradation.
The inventor herein has recognized the above-mentioned limitations of operating an engine at higher speeds and loads and has developed a method for operating an engine at higher speeds and loads such that the possibility of component degradation and engine emissions may be reduced. In one example, the method comprises operating an engine at a lean air-fuel ratio that reduces exhaust gas temperatures for a first portion of an operating period, and operating the engine at a rich air-fuel ratio that reduces exhaust gas temperatures for a second portion of an operating period when a temperature of an engine component is greater than a degradation threshold temperature of the engine component.
By operating the engine at both lean and rich air-fuel ratios that reduce exhaust gas temperatures, it may be possible to control and lower temperatures of engine components that are exposed to exhaust gases. In addition, by operating the engine rich and lean, it may be possible to allow a catalyst to operate at conditions where catalyst efficiency is high. For example, rather than operating with a solely rich air-fuel mixture during higher engine speeds and loads where catalyst hydrocarbon conversion efficiency may degrade over time, the excess oxygen present during lean operating conditions can be stored in a three-way catalyst to convert hydrocarbons to H2O and CO2 during conditions where the engine is operated rich. In this way, it may be possible to control and reduce temperatures of engine components while the catalyst operates at high efficiency.
The present description may provide several advantages. For example, the approach may improve engine emissions by continuing to convert hydrocarbons and NOx while the engine operates at higher engine speeds and loads. Specifically, excess oxygen in exhaust gases during lean operating conditions may be stored within a three-way catalyst and used at a later time to convert hydrocarbons to CO2 and H2O. Further, the approach can control and reduce temperatures of engine components that are exposed to engine exhaust gases by reducing exhaust gas temperatures that may be produced by the engine while the engine is operating at similar speeds and loads combusting substantially stoichiometric air-fuel ratios.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.